The present invention relates to a sample introduction assembly for loading small samples into analytical crucibles for subsequent analysis and particularly to a seal system which prevents the admission of contaminants.
In analytical furnaces for combusting relatively small (1 mg to 0.5 gram) samples of, for example, steel pins, chips, or the like, typically resistance or induction furnaces are employed. Graphite crucibles are employed for resistance heating of a crucible directly when placed between a pair of electrodes. Ceramic crucibles are employed in furnaces in which heating is by an induction field provided by an RF coil. In either furnace, it is necessary initially to outgas the crucible and assure no contaminant gases are mixed with the specimen gases during loading of the sample.
In several prior art systems, it is necessary to open the combustion chamber area after the outgassing to gain access to a crucible for insertion of a sample to be analyzed. In doing so, the crucible is exposed to atmospheric gases which can contaminate the crucible to an extent that the analytical results can be adversely effected. In order to prevent the introduction of contaminants, one solution has been to provide a sample loading mechanism which allows the introduction of a sample into a movable hopper which is subsequently sealed and the area purged with an inert gas. The jaws of the hopper are subsequently opened to allow admission of the sample into the crucible through an electrode assembly. U.S. Pat. No. 4,371,971 discloses such an apparatus which, although preventing a direct communication path with the atmosphere during admission of the sample, may allow a small amount of atmospheric gases to enter the combustion chamber during the sample loading operation through the dynamic seals on the movable jaw actuator. With analyzers designed to measure oxygen and nitrogen content of a specimen, even miniscule amounts of atmospheric oxygen and nitrogen adds inaccuracy to analytical results, particularly for low concentration samples. Even with sealed sample dropped mechanisms where linear acting pistons move through radial seals, gases trapped in imperfections on the shaft surface are introduced to the analytical specimen, degrading the precision and accuracy of the measured amount of oxygen and nitrogen. Also, with time, atmospheric leakage increases as dynamic seals wear due to high cyclical use.
Accordingly, there exists a need for an improved sample introduction system in which contamination from atmospheric contaminants can be eliminated.